1. Field of the Invention
The present invention relates to a radio terminal, a radio communication system, and a radio communication method. Specifically, the radio communication system includes a base station and a plurality of radio terminals and allowing radio communication to be performed by using a predetermined radio band, and the plurality of radio terminals each having a radio interface for performing radio communication with the base station.
2. Description of the Related Art
Conventionally, a radio communication scheme using carrier sense multiple access with collision avoidance (CSMA/CA) has been known to be employed in a radio network including a base station and a plurality of radio terminals.
In general, the order in which the radio terminals transmit their packets is determined randomly. Alternatively, the order in which the radio terminals transmit packets is determined by scheduling in the radio network. In scheduling, time (slot) during which a radio terminal transmits a packet is determined for each of the radio terminals. This reduces collision probability of packets transmitted by the radio terminals, allowing radio resources to be effectively used.
The following techniques have been proposed as a technique for the radio communication scheme using CSMA/CA.
In a first technique, distributed coordination function (DCF) is defined as a communication protocol for a wireless LAN (see, for example, IEEE Standard 802.11, 1999 (R2003) and its amendments, IEEE Press). DCF is a protocol for allowing radio terminals to autonomously and distributedly determine their timings to transmit packets.
In a second technique, enhanced distributed coordination access (EDCA) is defined as a communication scheme using quality of service (QoS) on the basis of DCF (see, for example, I. Aad, P. Hofmann, L. Loyola, J. Wdmer, “Self-organizing 802.11-compatible MAC with Elastic Real-time Scheduling,” in proceedings of IEEE MASS 2007, October 2007, Pisa, Italy).
In a third technique, the scheduling is performed by a representative one of the plurality of radio terminals (see, for example, IEEE Standard 802.11e, 2005). Specifically, in the third technique, a communication protocol further enhanced based on IEEE 802.11e used in the second technique is proposed. In the third technique, the representative radio terminal performs scheduling of a packet transmission order in a real-time application. The representative radio terminal transmits a packet for notification of the packet transmission order. Each of the radio terminals monitors the packet for notification of the packet transmission order.
Meanwhile, a radio terminal having a power-saving mode to reduce its electric power consumption is known. In the power saving mode, the electric power consumed by the radio terminal is reduced by performing switching from a wake-up state to a sleep state. In the wake-up state, packets can be transmitted and received. In the sleep state, a radio interface in the radio terminal is turned off.
The following techniques have been proposed as a technique for reducing the electric power consumed by the radio terminal.
In a fourth technique, the radio terminal estimates a timing at which a packet will be transmitted or received (called a transmission/reception timing below), and switches from the sleep state to the wake-up state at the transmission reception timing thus estimated (see, for example, U.S. Pat. No. 7,181,190 “Method for maintaining wireless network response time while saving wireless adapter power”).
In a fifth technique, a MAC protocol based on time division multiple access (TDMA) is proposed. Specifically, the radio interface in the radio terminal is turned off in slots other than the slot used to transmit or receive packets. (See, for example, Zhihui Chen and Ashfaq Khokhar, “Self Organization and Energy Efficient TDMA MAC,” 2004 First Annual IEEE Communications Society Conference on Sensor and Ad Hoc Communications and Networks, 2004. IEEE SECON 2004.)
In a sixth technique, each of the radio terminals monitors a period in which a different radio terminal transmits or receives packets (called a packet transmission/reception period below). Thereby, timings of switching from the sleep-state to the wake-up state are scattered, and accordingly delay is prevented. (See, for example, Alessandro Giusti, Amy L. Murphy, and Gian Pietro Picco, “Decentralized Scattering of Wake-up Times in Wireless Sensor Networks,” in Proc. of EWSN 2007.)
With the above techniques, however, it is difficult to accomplish both of effective use of the radio resources and reduction in electric power consumption. Specifically, as mentioned above, the radio terminal turns off the radio interface in the sleep state. As a result, the radio terminal in the sleep state cannot monitor the packet for notification of the packet transmission order. Likewise, the radio terminal in the sleep state cannot monitor the packet transmission/reception period of a different radio terminal.
To be more specific, in point coordinating function (PCF) according to the first and second techniques, timings to perform polling are not defined exactly. Consequently, the radio terminal cannot switch to the sleep state in a time period between the start of a period controlled by PCF (a contention free period (CFP)) and the completion of transmission or reception of packets.
In the third technique, the radio terminal in the sleep state cannot monitor the packet for notification of the packet transmission order.
In the fourth to sixth techniques, the radio terminal in the sleep state cannot discover completion of radio communication of a different terminal. This might lead to a case where the radio resources are not effectively used.